The present invention relates to improvements in segmented ring magnets. More particularly, it provides a segmented permanent ring magnet which is tunable to provide a preselected transverse magnetic field within its interior.
The development of nuclear magnetic resonance (NMR) as a tool for medical and biological imaging has generated a new interest in magnet development. NMR imaging systems have four major parts: a bias field magnet, gradient coils for spatial location, an R-F coil which causes a short term perturbation of the nuclear magnetic moments of a specimen material within the magnet, and a receiver. The bias field magnet provides a relatively large, preselected field, e.g., a substantially uniform transverse field, within the interior of the magnet. The bias field promotes alignment of the nuclear magnetic moments of nuclei in the specimen material parallel to the applied field. This alignment causes a change in the magnetic state of the material. Significant magnetic fields. e.g., one to five kilogauss, generally are needed to achieve the desired alignment.
Typically, the X, Y, and Z gradient coils are used to cause directional perturbations in the magnetic field, e.g., the X-coil causes a gradient in the X-direction. These gradient coils are pulsed, individually or collectively, at intervals set by a sequencer to provide the spatial information necessary for image reconstruction. The gradient coils cause perturbation in the magnetic field pattern so that only one field location, e.g., a plane, is unperturbed. The NMR receiver scans the magnetic field and obtains the spatial information necessary for reconstruction of an image by the location of the unperturbed regions.
The R-F coil is turned on by the sequencer for a short period after alignment of the magnetic moments of the nuclei by the bias field. The R-F coil causes a perturbation or excitation in the magnet field which displaces the nuclear magnetic moments of the nuclei from their aligned position. Once the excitation field from the R-F coil is removed, the nuclear magnetic moments of the nuclei revert back to their aligned position in response to the applied bias field. The receiver, which measures the strength of the signal which results from the realignment of the nuclear magnetic moments over time, is used to determine the time constant of this reversion. The differences in the strength of the signal and the time constant, factors which vary from material to material, make it possible to identify the different properties of the specimen materials within the person or sample being analyzed. The R-F coil may also be used as the receiver.
A major problem of such prior NMR imaging systems for use with a human specimen is the construction of the bias magnet. This magnet it to have sufficient field strength to promote alignment of the magnetic moments of the nuclei and yet have a large bore size. The large bore is necessary to accomodate a major portion of the human body, at least the head and shoulders, within the field. If other properties of the magnet are kept constant, the magnetic field strength decreases with increasing bore size, so the design problems associated with a high field, large bore magnet are significant.
The problems of producing a bias field sufficient for biological applications across a large bore led the developers of the first NMR imaging systems to use superconducting bias magnets. While superconducting bias magnets can easily produce the needed field, there are several problems with the use of these magnets. First, expensive cryogenic equipment is required to maintain the superconducting state. Second, if the superconducting material becomes conductive, e.g., by a rise in temperature, the magnetic field strength deteriorates rapidly. These complications have led to attempts to develop NMR imaging systems using permanent magnets. For example, Fonar Corporation uses a permanent magnet system which produces a dipole magnetic field. The perturbations in this dipole magnetic field can be reconstructed by a computer to provide the information sought. Oldendorf Magnet Technology has constructed a model magnet for an NMR imaging system of a ferrite permanent magnet material. However, neither of these systems yields imaging which is sufficiently detailed for many medical applications.
The rare earth alloys and related materials show promise for use in the construction of bias field permanent magnets for NMR imaging systems. These materials have essentially linear behavior in the second quadrant of the plot of the B-H relationship, which is the graph of the demagnetization-applied field strength relation. These materials do not have a large hysteresis loop in the region of interest, e.g., in contrast to the hysteresis loop observed for soft iron. Therefore, once these materials have been magnetized, the strength of the field and the orientation of the easy axis of magnetization, i.e., the direction of the magnetic field of the material, stay substantially constant over a wide variety of conditions. Materials which exhibit these properties include rare earth cobalt alloys, ceramic ferrite alloys, and rare earth iron alloys. These materials normally are manufactured by placing a finely ground powder of the material in a mold and simultaneously subjecting the powder to pressure and an applied magnetic field. This procedure forms the powder into a solid block having a substantially aligned easy axis of magnetization.
There are several problems associated with the foregoing use of the rare earth alloy-like materials as a source of the bias field of an NMR imaging device. First, the field strength from these magnets is not as great as can be obtained from superconducting magnets. This means that the size of the bore is more critical. Second, the comparatively high cost of these materials, approximately $30-45/pound, limits the size and weight of the magnet. If the permanent magnet material weighs more than about 10,000 pounds, the cost of the magnet may be prohibitive for use in NMR imaging systems. At weights lower than 5,000 pounds, the bore must be made too small to be useful for human imaging or the field is too low. The approximate 10,000 pound limit appears to provide a commercially reasonable compromise between cost and field strength. Third, the sizes of the pieces of the rare earth alloy-like material which can be manufactured and magnetized using current techniques are substantially smaller than the required size of this large magnet. Therefor, a magnet is formed of a plurality of segments. Each segment is, in turn, formed of a plurality of pieces of magnetic material. In addition, non-uniformities stemming from the magnetization process can cause harmonic inaccuracies in the resultant magnetic field.
Accordingly, an object of this invention is to develop a tunable segmented ring magnet of a permanent magnetic material which produces a preselected transverse magnetic field in the interior of the ring structure. Another object is to provide a method of tuning a segmented permanent ring magnet to achieve a preselected transverse field in the interior of the ring. A further object is to provide a method of fabricating a magnet segment having a preselected easy axis of magnetization. A still further object is to provide a permanent magnet formed of a rare earth alloy-like material which is useful as a bias magnet in an NMR imaging system. These and other objects and features of the invention will be apparent from the following description and the drawing.